Several processes have been in use over the years for drying green, i.e., moist, iron ore pellets, e.g., hematite, magnetite or limonite. The objective of these processes is to remove residual moisture so as to produce a strong fired pellet having maximum abrasion and breakage resistance as adjudged by crushing tests, optimum porosity and, where stored in cooler climates, good resistance to repeated freezing and thawing. In treating certain ores the process should also provide optimal oxygenation, since poor strength may otherwise result in the case of magnetite pellets where oxidation to Fe.sub.2 O.sub.3 is not complete, leaving magnetite cores in the center of the pellets.
Prior methods employed in drying iron ore pellets will now be described briefly by way of example in connection with the drying of magnetite pellets obtained from taconite. It should be understood, however, that although the present invention is described in connection with a particular ore, it is not limited to specific apparatus or processes described.
For the last 45 years the beneficiation of magnetite-containing rock has consisted of crushing, grinding and milling the ore. The specific operation consists of separating the desired material from the gangue (waste) material through hydraulic separation, magnetic separation, and by chemically treating the ore to further enhance the separation of the ore from the waste rock.
The material separated from the waste material is called concentrate. The total iron may range from 65% to 69% or other economically practical value. The concentrate is generally described as a powder with the general size that can pass through a screen of a selected size. The screen usually used is a U.S. Standard Tyler Screen of 325 and 500 mesh to the inch. The 500 mesh screen has openings about 27 microns in diameter.
Some of the general size descriptions might be 85% minus 325 mesh and 75% minus 500 mesh as an example. The percentage values correspond to the amount of grinding necessary to liberate the desired product from the waste product. The grinding, milling and treatment of the ore generally occur in a section of the plant called the concentrator, hence the name concentrate.
The concentrate is generally piped in an aqueous slurry of 60% solids to a vacuum filter. The vacuum filter removes most of the water from the slurry. The resulting product is called a filter cake with generally less than 10% water. The amount of water is controlled by the efficiency of the filtering operation and also by the size of the particles in the concentrate. The concentrate (filter cake) is generally conveyed to storage bins before being fed into a disk or drum balling device.
The concentrates have additives to improve the balling, firing or chemical composition of the product once it has been fired. Some of the common additives are bentonite clay, limestone in the form of calcium hydroxide if fluxed pellets are produced, and sometimes an organic binder.
The balling of concentrate is accomplished in a process in which the material is rolled in stages that increase the size of the pellet by applying a layer of concentrate upon a smaller pellet until the pellet reaches the desired size. The product from a balling drum is screened to selectively size the product. The undersized material is circulated back into the balling drum. The circulated material is called seed pellets. The balling action applies the concentrate to minimize interstitial spaces, hence smaller particles are forced between larger particles. The mixture of particle sizes makes a pellet of maximum density. The additives also fill the interstitial spaces and often provide a pathway for the gradual removal of water from the inside of the pellet. Pathways are also provided for oxygen to enter the inside of the pellet during the firing of the pellet. Knowledge of the removal of water from the inside of pellets is necessary to appreciate the contributions that the present invention provides towards the firing of magnetite pellets. An adequate preliminary description of the equipment and the mineral beneficiation process has been provided. It is also necessary to describe the physical and chemical changes in each section of a pelletizing machine.
The prior drying process and some of the limitations of that system which negatively impact on the next stage of the pelletizing process (the firing of the pellets) will now be described. It should be noted, however, that even a detailed explanation of the physical changes of the product is an oversimplification of a complex process.
The finished pellets are screened and placed on conveyor pallets each having grate bars at its bottom that holds the pellets as they travel through the furnace. The pellets are placed gently on the pallet grate bars to form a level bed of pellets at a depth that has been established through practical experience. The depth is usually about 15 inches or more in thickness. Quite frequently, a layer of recently fired pellets is first placed upon the grate bars to form a layer of fired pellets about 3 inches thick. The fired pellet layer is called a hearth layer. Each pallet is part of an endless track conveyor about 300 feet long and often 8 to 12 feet wide. One common conveyor is called a traveling grate machine. The conveyor is part of and contained for the most part within the drying, firing magnetite conversion and cooling zones of a furnace.
There are zones or sections of the furnace named to describe the process that occurs in each zone of the furnace. Generally, the first zone of a travelling grate furnace is the updraft drying zone. The present invention is used in this section of the furnace, as well as the next zone called the downdraft drying zone (DDZ).
As an example, consider that a hearth layer of fired pellets 3 inches deep is placed upon the pallet grate bars. A layer of finished pellets 15 inches deep is then placed upon the hearth layer, making a total depth of 18 inches. The hearth layer is dry and the pellets in the finished pellet layer contain 10% water. The grate bars are aligned on the pallet to provide openings about 1/4 inch wide to permit hot air to flow through the openings.
The updraft drying zone of the furnace consist of windboxes beneath the travelling grates. Each windbox is designed to provide a reasonably airtight seal to force air under pressure up through the bed of pellets that is on the travelling grate. A large quantity of air is directed up through both the hearth layer and the layer of finished pellets. The air temperature is generally 600.degree. F. to 850.degree. F. This description applies to a continuously travelling grate machine that is in equilibrium for temperature and airflow. As an example, consider an 8 ft. wide by 8 ft. long windbox. Assuming the grates travel 96 inches a minute, any pellets are above a windbox for one minute. Hot air is forced up through the pellet bed by a forced draft fan. Sufficient upward velocity and static pressure is maintained to establish an upward airflow. The hot air blowing by the finished pellets evaporates surface water while water inside the pellets slowly evaporates. Some of the heat energy warms the pellets, but most of the heat is used to evaporate water on and within the pellets. The heating and evaporation proceeds from the bottom up through the pellet bed. The transfer of heat travels slowly up through the pellet bed. The evaporation of water cools the air by an amount of energy called the heat of vaporization. The heat transferred to solid masses such as the pallet frames and the hearth layer is called sensible heat transfer.
It is necessary to understand some of these physical changes to evaluate the potential attributes of my invention. Moist air travelling up through a bed of cold pellets is eventually cooled to the dewpoint temperature so that water vapor condenses on the cool pellets, thereby increasing the water content of the pellets. Air travelling up through the pellet bed also carries moisture entirely through the pellet bed. The amount of water removed is consistent with the moisture carrying capacity of the air. The amount of water vapor present is the 100% relative humidity value for the temperature that the air leaves the pellet bed. Water vapor removed in this manner is the primary way that water is removed from the pellet bed. Some of the water evaporated from the lower half of the pellet bed is, however, merely transferred by the condensing action to the cooler pellets in the upper portion of the pellet bed. The pellets on the top of the pellet bed increase in water content by the condensing of water vapor upon their surface so that pellets that originally had less than 10%, now will contain over 12% water, mainly on the surface of each pellet.
The volume of water removed in the updraft drying zone (UDZ) of the furnace probably exceeds 40 gallons of water per minute. The water removed passes through the top of the pellet bed as water vapor. Forty gallons per minute corresponds to 50% of the water contained in pellets entering the drying zone at a rate of 200 tons per hour.
The cooler pellets near the top of the pellet bed are at the dewpoint temperature. These pellets help control and establish the dewpoint of the moist air travelling upward through the bed of pellets. Essentially the 40 gallons of water removed as water vapor came from the lower section of the pellet bed.
At the end of the UDZ, the pellets at the bottom of the pellet bed are at the temperature and water content correct for the next stage of the firing process prior to the actual firing process. However, in the sequence being described they will not be fired until the end of the firing sequence. At the end of the UDZ the pellets in the top 4 inches of the pellet bed still are wet (over 10% water) and these are the pellets that are to be fired in the final zone, the downdraft firing zone (DFZ) because the DFZ fires the top of the pellet bed first. Following the UDZ is the downdraft drying zone (DDZ) in which the air direction is down onto the pellet bed. The top pellets entering this zone are wet with a water content exceeding 10%. For a depth of 5 or 6 inches the pellets are wetter than when they were initially placed on the pallets. The thrust of air directed upon the pellet bed and the suction of the waste gas fan in the DDZ provide energy to draw air down through the bed of pellets. The pellets are in the downdraft drying zone of the furnace for only about 2 minutes.
Very little drying takes place in the DDZ of the furnace. This becomes clear when one considers how hard it is to suck air downwardly through 15 inches of pellets, especially when the top 6 inches are wet. Any water that is evaporated expands to steam and artificially increases the volume of gas travelling through the bed of pellets. This is an important factor upon which the present invention is based. The present invention will effectively minimize the problem caused by inadequate drying that occurs in both the updraft and downdraft drying zones of pelletizing furnaces.
Following the DDZ, the pellets enter the downdraft firing zone (DFZ) with no delay. The temperature in the DFZ is typically 1600.degree. F. to 1800.degree. F. A waste gas fan draws the heated air and combustion gasses through the pellet bed. Pellets that are wet to a depth of about 6 inches from the top of the bed with about 10% water are exposed to hot air (1800.degree. F.) which flows downwardly through that mass of pellets.
The balling drum additives such as bentonite clay, organic binder, limestone or a similar basic oxide present in the pellets, provide pathways for water vapor to escape. The limestone is added when fluxed pellets are desired. While probably providing pathways for water vapor removal, it is likely that the limestone will maintain a higher moisture level than what would be present without the limestone. If adequate amounts of additives are not present to provide a pathway for steam to escape the pellets' interior, the pellets may explode and break off part of the outside of the pellet. This unfavorable characteristic is called spalling. With an adequate amount of additive present, however, the water in the pellet is escaping at the time that it would be desirable for oxygen to penetrate to the center of the pellet and begin the conversion of magnetite to hematite reaction. If complete conversion does not take place, a magnetite core results. magnetite cores can be caused by introducing pellets with too much water into the firing zone of the furnace. The outer layers of the pellets are often sealed through grain growth, thus eliminating the possibility of oxygen reaching the center of the pellet. This is another way that magnetite cores can be produced. The magnetite cores contribute to breakage problems in transportation or inhibit proper blast furnace conversion.
In view of these and other deficiencies, there exists an important need for an improved ore pellet drying process that is not subject to the aforementioned problems and shortcomings.
It is therefore one objective of the present invention to provide an improved ore drying process suited for drying pellets of magnetite, hematite, limonite or other ores in which the pellets have improved strength, abrasion and breakage resistance.
Another object of the invention is to provide fired pellets with the aforesaid advantages which also have optimum moisture content, porosity and resistance to repeated freezing and thawing when fired pellets are produced.
A further object of the invention is to provide an improved ore drying process for hematite, magnetite or limonite wherein a more uniform drying is accomplished throughout all portions of the bed of pellets being dried due to the elimination or reduction of a moisture gradient between the top and bottom surfaces of the pellet bed and to eliminate or reduce the presence of magnetite cores in fired magnetite pellets.
These and other more detailed and specific objects of the present invention will be better understood by reference to the following figures and detailed description which illustrate by way of example of but a few of the various forms of the invention within the scope of the appended claims.